The present invention concerns a crossbar switch with a number of rows and a number of columns crossing the rows at crossing points, in which: a) an input signal is supplied to each row via a row input; b) controllable switch elements are arranged at the crossing points via which an input signal supplied to a row is connected through to the column crossing this row at this crossing point when a switching signal is supplied to the respective crossing point, and c) whereby a switched input signal is tapped as an output signal to each column via a column output.
It is the object of a crossbar switch to interconnect a number of input signals to a number of outputs according to arbitrary specifications. The number of input signals is normally different than the number of output signals. In order to achieve such a switching capability, switch elements are arranged at crossing points, whereby each switch element can be switched individually and thus independent of the other switch elements. Consequently, a number of control lines are necessary for correct activation of the crossbar switch; this number is equal to the product of the number of rows with the number of columns.
Crossbar switches of this type are, for example, distributed by the company Analog Devices as integrated crossbars with the type designations AD8108 through AD8111 as well as AD8114 and AD8115.
In these Analog Devices' crossbars, passively operated field effect transistors are used as switch elements. These crossbar switches therefore have only limited channel numbers of, for example, 8×16, 16×8 or 16×16 input and output signals. These components also exhibit very high rise of the transmission loss at high frequencies, such that it is not possible to interconnect these components as sub-matrices into a larger matrix.
Crossbar switches are also known in which pin diodes are used as high-frequency switches. Given very high frequencies (for example, above 100 MHz), the lock damping of a pin diode connected as a series switch is, however, no longer sufficient, such that a three-fold structure (series-short-series switch) must be chosen. In such a three-fold structure, the control current must be fed to the high-frequency connections of the switch elements. The necessary expenditure of HF chokes and blocking capacitors is therefore significant and connected with secondary effects. In practice, with such three-fold structures, no crossbar switches can therefore also be realized with numbers of high channels.
Switches are also conceivable in which a large number of switch elements possess a common star point. However, these concepts exhibit a serious disadvantage, since, at the star points, the sum of the capacitiances of all switch elements is switched in parallel. Together with the source or load impedance of the connected circuits, a limit frequency therefore results that is inversely proportional to this total capacitance.
The attempt to choose a lower source and load impedance leads in return to a larger absorption in the series resistance of an interconnected switch element. The attempt to counteract this effect with a larger chip area of the switch elements in turn leads to a higher capacitance. Given a predetermined frequency bandwidth and predetermined transmission losses, there is therefore a maximum number (dependent on the switch technology) of junctions that can still be realized. Very high node counts (for example, 64 nodes or more) at high limit frequencies of, for example, 100 MHz cannot be realized.